Flare-stack

ABSTRACT

A flare-stack suitable for burning off waste gases comprises: A. A HONEYCOMB BURNER ELEMENT ON SUPPORTS B. FLARE-STACK WALLS PROJECTING VERTICALLY UPWARDS FROM THE EDGES OF THE BURNER ELEMENT, THE FLARE-STACK WALLS AND BURNER ELEMENT BEING ENCLOSED BY C. A DRAUGHT FENCE AROUND THE BASE OF THE FLARE-STACK WALLS. During operation of the flare-stack, air flows through the draught fence, below the flare-stack walls and into the combustion air tubes of the burner.

StatesPatent [191 Desty et a1.

FLARE-STACK [75] Inventors: lDenis Henry Desty, Weybridge;

Christopher John Young, Horton, both of England [73] Assignee: TheBritish Petroleum Company Limited, London, England [22] Filed: Aug. 9,1972 [21] App]. No.: 279,002

[30] Foreign Application Priority Data Aug. 12, 1971 Great Britain3786/71 [52] US. Cl. 431/202, 431/328 [51] Int. Cl. F23d [58] Field ofSearch 431/202, 328, 5

[56] References Cited UNITED STATES PATENTS 2,971,605 2/1961 Frost et a1431/202 3,504,994 4/1970 Desty et al 431/328 3,703,349 11/1972 Straitz431/202 FOREIGN PATENTS OR APPLICATIONS 1,227,524 4/1971 Great Britain431/328 Primary Examiner-Carroll B. Dority, Jr. Attorney, Agent, orFirmMorgan, Finnegan, Durham & Pine [57] ABSTRACT walls. Duringoperation of the flare-stack, air I flows through the draught fence,below the flare-stack walls and into the combustion air tubes of theburner.

2 Claims, 4 Drawing Figures FLARE-STACK This inventionrelates to a flarestack for disposing of waste combustible gases and in particular relatesto the disposal of waste gases from refineries and chemical plant.

In the refining of crude petroleum there are usually produced and/orseparated from the oils, gases comprising hydrocarbons e.g. butane,propane. it is not always possible to sell or otherwise make use ofthese combustible gases at low pressure and it is customary to burn themoff in a flare stack. in the case of sour gases, i.e., containingsulphur, ground level flaring is impractical and an elevated flare stackis used to aid the dispersion of any oxides of sulphur that may beformed during combustion.

During emergency flaring, very large quantities of combustible gases,often of the order 200 tons/hour have to be dealt with. It is alwaysdesirable and frequently essential, because of legislation, to minimiseand if possible to eliminate the amount of smoke emitted by a flare. Toreduce the amount of smoke emission under these conditions, steaminjection into the flare stack may be used to improve combustion.

A similar problem occurs in oil fields where the oil produced often hasassociated with it large quantities of hydrocarbon gases. Any excess ofgas is dealt with, as in the case of refineries by flaring-off the gas.A suitable flare stack for this purpose is described in our co-pendingBritish Patent application No. 12068/71.

However for steady state flaring of sweet gases, factors of cost, andthe noise and visibility of the flares render the steam injected flarestacks less suitable for this purpose and it is therefore normal toprovide an enclosure for flaring which is mounted at ground level.

According to the invention a burner element for use in a ground firedflare stack comprises a plurality of combustion air tubes adapted toconvey combustion air to the combustion zone, each combustion air tubehas a cross-sectional area of not less than 1.0 cm where it opens intothe combustion zone and the bores of the combustion air tubes accountfor at least 25 percent of the area adjacent to the combustion zone, thetubes passing through a fuel chamber divided into:

a. a fuel outlet space which communicates with the combustion zone andwhich contains packing which provides a high resistance to the flow ofgaseous fuel, and

b. a fuel inlet space which is adapted for connection to a fuel supplyand which contains no packing material so that it provides a lowresistance to the flow of fuel, whereby, during the use of the burner,air flows through the combustion air tubes into the combustion zonewhere it reacts with the fuel which flows through the inlet and outletspaces and finally into the combustion zone.

Preferably the combustion tubes are of 1 inch diameter and spaced at 1%inch separation of centres and of length 4 inches.

The burner element described in the last preceding paragraph has thespecial property that it can burn a wide variety of gases, e.g., butane,methane and liquids, e.g., liquid petroleum gas, by a diffusion flamemechanism. Also, the low back pressure created by the burner is anadvantage in situations where the gas is only available at lowpressures.

For gaseous fuels, the fuel outlet space provides a resistance to flowwhich is high in comparison with the resistance of the unpacked spaces.(This implies that no low resistance channels are left, e.g., around thecombustion air tubes in the packed spaces). This high resistance servesto retain gas in the fuel inlet space while even fuel distribution isachieved. The packing is preferably a porous material, e.g., sand whoseparticle size and particle density is such as to produce the requiredhigh resistance to fuel flow.

The packing is conveniently supported on a plate which extends acrossthe fuel chamber and which plate permits passage of fuel. Examples ofsuch plates include gauzes, perforated plates and plates which providean annular fuel passage around each of the combustion air tubes. Incertain cases the packing may have sufficient mechanical cohesion torender the use of a plate unnecessary.

The invention also includes a gaseous fuel burner element as describedabove which also incorporates one or more pilot tubes which terminate inthe fuel outlet space, the pilot tubes being so sized that, during use,they supply enough fuel to provide a pilot flame for reignition.

Preferably the pilot tube, or each pilot tube when there is more thanone, terminates near the boundary between the fuel outlet space and theadjacent unpacked zone.

In the case of liquid fuel combustion, the fuels used are those havingboiling points at or below room temperature. Heat transfer from thesurrounding air via the tube bank heat exchanger formed by thecombustion air tubes enables the liquid fuel to be vaporised, the fuelthen passing through the packing as described previously for gaseousfuels.

The following two constructions are particularly suitable for use at theinlet end of the combustion air tubes:-

CONSTRUCTION A The combustion air tubes are secured in fluid tightmanner into holes in an air inlet zone plate which forms one wall of thefuel inlet space.

CONSTRUCTION B The combustion air tubes have a conformable polygonalcrosssection, e.g., equilateral triangles, square or regular hexagonsand, at the inlet end of the combustion air tubes, the walls of thetubes are secured to one another in fluid tight manner.

The burner element can form part of flare stacks used for the burningoff of residual gases.

According to another aspect of the invention there is provided a flarestack suitable for burning off waste gases comprising a. a diffusionflame burner element (as described above) on supports b. flare stackwalls projecting vertically upwards from the edges of said burnerelement, the flare stack walls and burner element being enclosed by c. adraught fence around the base of the flare stack walls whereby duringoperation of the flare stack air flows through the draught fence, belowthe flare stack walls and into the combustion air tubes of the burnerelement.

Preferably the length of a side of the total burner element is not lessthan 5 or greater than 30'. Burner elements having dimensions smallerthan this are unsuitable for flaring off refinery waste gases as theyare not capable of dealing with the quantities generally used. Burnerelements greater than about 30 in length give rise to constructionaldifficulties.

Preferably the height from the ground to the burner element isone-fourth to one-third of the length of a side of the total burnerelement. This enables an adequate supply of air to be maintained to theburner element to enable smoke-free combustion of the fuel.

In a preferred embodiment of the invention the flare stack walls areconstructed from steel reinforced refractory concrete, and the draughtfence is a slat arrangement constructed from glass fibre reinforcedconcrete.

Preferably the diffusion flame burner element is of a modularconstruction, i.e., the element is made up of a suitable number of smallunits. This gives advantages in assembling the stack and in themanufacture of the burner element.

According to another aspect of the invention there is provided a methodfor disposing of low pressure residual gases or volatile liquids whichcomprises burning off the gases in a flare stack as hereinbeforedescribed in the Specification by passing the gases through a burnerelement fitted with a plurality of combustion air tubes adapted toconvey combustion air to the combustion zone, each of which tubes has across-sectional area of not less than 1.00 cm where it opens into thecombustion zone and the bores of the combustion air tubes account for atleast 25 percent of the area adjacent to the combustion zone, the tubespassing through a fuel chamber divided into:

a. a fuel outlet'space which communicates with the combustion zone andwhich contains packing which provides a high resistance to the flow ofgaseous fuel, and

b. a fuel inlet space connected to a fuel supply and which contains nopacking material so that it provides a low resistance to the flow offuel, air flowing through the combustion air tubes into the combustionzone where it reacts with the fuel which flows through the inlet andoutlet spaces and finally into the combustion zone.

The invention will now be described by way of example, with reference tothe diagrammatic drawings accompanying this specification in which:

FIG. 1 is front view of the ground fired flare stack.

FIG. 2 is a perspective view, with part cut away of the burner unit ofthe flare according to the preferred embodiment of the invention.

FIG. 3 is a vertical cross-section through the burner element shown inFIG. 2.

FIG. 4 is a perspective view of the burner element showing the method ofconstruction from modules.

A front view of a flare stack is shown in FIG. I. The fuel is fed intothe stack by means of a fuel inlet pipe 1 which passes through thedraught fence 3 via a hole 2 and is piped into the burner element. Airpasses through under the flare stack walls 4 into the combustion zone(not shown). The burnt gases then pass to the atmosphere along the flarestack walls 4.

The burner element shown in FIGS. 2 and 3 comprises a fuel chamber 5through which a plurality of combustion air tubes 6 pass. The fuelchamber is divided into a fuel inlet space 7 and a fuel outlet space 8which is packed with porous material, preferably sand to increase itsflow resistance, said packing being supported by means of a partition 9.Each of the combustion air tubes 6 passes through a hole in thepartition 9 and the size of the hole is such that an annular fuel duct10 is formed around each of the combustion air tubes.

In the use of the burner element, fuel enters the fuel inlet space 7 viathe fuel supply line 12 and it passes through the interstices of thecombustion air tubes 6. From the fuel inlet space 7 the fuel passesthrough the packed fuel outlet space 8 into the combustion zone. Sincethe packing offers a relatively high resistance to the fuel flow and theinterstitial space of the fuel inlet space 7 offers a relatively lowresistance the construction encourages a uniform supply of fuel into thecombustion zone.

FIG. 4 shows a cross-section of a flare stack incorporating modularburner elements 16. In use, the fuel enters the fuel inlet pipe 1 andbranches off via module feed pipes 17. Fuel then enters each burnerelement module through suitably arranged inlets l3 and 14. The totalburner element stands on supports 15.

A flare stack using a burner as described above was tested using butaneand propane as the fuel. The burner element comprised combustion airtubes each having a diameter of 1 inch, the overall dimensions of theburner element being 12 foot square by 4 inches deep. It was fitted witha perforated plate and the top 1 inch was packed with coarse sand. Theburner element was incorporated in a flare stack. The stack walls wereconstituted of steel reinforced refractory concrete and the draughtfence of glass fibre reinforced concrete. The height from the burnerelement to the top of the stack was 15 feet.

What we claim:

1. A flarestack for burning waste gases, comprising:

A plurality of modular burner elements constituting a burner unit; eachof said burner elements having a plurality of combustion air tubesadapted to convey combustion air to a combustion zone, each of saidcombustion air tubes having a cross-sectional area of not less than 1.0cm. where it opens into said combustion zone and the bores of saidcombustion air tubes accounting for at least 25 percent of the areaadjacent to said combustion zone, said combustion air tubes passingthrough a fuel chamber divided into a fuel outlet space whichcommunicates with said combustion zone and which contains packing toprovide a high resistance to the flow of gaseous fuel, and a fuel inletspace adapted for connection to a fuel supply and which contains nopacking material to provide a low resistance to the flow of fuel;flarestack walls projecting vertically upwardly from the edges of saidburner unit; the length of a side of said burner unit being not lessthan 5 feet or greater than 30 feet; the height from the ground to saidburner unit being a quarter to a third of the length of a side of saidburner unit; and a draught fence enclosing the base of said flarestackwalls and said burner unit, whereby during operation of said flarestack,air flows through said draught fence, below said flarestack walls andinto said combustion air tubes of said burner unit. 2. A flarestack asclaimed in claim 1, wherein each of said combustion air tubes has across-sectional area of 5.0 cm. where said tube opens into saidcombustion

1. A flarestack for burning waste gases, comprising: A plurality ofmodular burner elements constituting a burner unit; each of said burnerelements having a plurality of combustion air tubes adapted to conveycombustion air to a combustion zone, each of said combustion air tubeshaving a crosssectional area of not less than 1.0 cm.2 where it opensinto said combustion zone and the bores of said combustion air tubesaccounting for at least 25 percent of the area adjacent to saidcombustion zone, said combustion air tubes passing through a fuelchamber divided into a fuel outlet space which communicates with saidcombustion zone and which contains packing to provide a high resistanceto the flow of gaseous fuel, and a fuel inlet space adapted forconnection to a fuel supply and which contains no packing material toprovide a low resistance to the flow of fuel; flarestack wallsprojecting vertically upwardly from the edges of said burner unit; thelength of a side of said burner unit being not less than 5 feet orgreater than 30 feet; the height from the ground to said burner unitbeing a quarter to a third of the length of a side of said burner unit;and a draught fence enclosing the base of said flarestack walls and saidburner unit, whereby during operation of said flarestack, air flowsthrough said draught fence, below said flarestack walls and into saidcombustion air tubes of said burner unit.
 2. A flarestack as claimed inclaim 1, wherein each of said combustion air tubes has a cross-sectionalarea of 5.0 cm.2 where said tube opens into said combustion zone.